It has been proposed to use UWB using a quasi-millimeter waveband of 22 to 29 GHz as automotive radars or portable short range radars (SRR).
As an antenna of a radar device used within the UWB, not only a radiation characteristic thereof must be a wideband, but also it is necessary for the antenna to have a compact size and a thin, flat structure in consideration of the fact that it is provided, for example, in a gap between a vehicle body and a bumper at the time of being mounted on a vehicle.
Further, as this antenna, low-loss and high-gain are required in order to carry out exploration with weak radio waves specified by the UWB, and to suppress wasteful electricity consumption so as to be battery-driven, and therefore, it is necessary for the antenna to be easily set in an array.
Furthermore, as this antenna, it is desirable that a feed unit for antenna elements can be manufactured by a printing technology in order to realize low-cost manufacturing.
Moreover, as regards radars, it is desired to use circular polarization whose cross polarization component is small in order to be free of the influence of a secondary reflected wave.
As described above, a band of 22 to 29 GHz is to be used for UWB radars. However, a RR prohibited band (23.6 to 24.0 GHz) for protecting passive sensors of radio astronomical or earth exploration-satellite services (EESS) is included in this band.
In 2002, the FCC (Federal Communications Commission) of USA has disclosed the rule that an average power density is −41.3 dBm or less, and a peak power density is 0 dBm/50 MH at 22 to 29 GHz in the following Non-Pat. Document 1.
In this rule, it is stipulated that a wave angle side lobe is reduced to be −25 dB to −35 dB every several years in order to suppress radio interference onto the aforementioned EESS as well.
Non-Pat. Document 1: FCC 02-48 New Part 15 Rules, FIRST REPORT AND ORDER
However, in order to achieve this, a dimension in a vertical direction of an antenna for use in a UWB radar is made larger, and it is assumed that it is difficult to mount the antenna onto a general passenger vehicle.
Therefore, the FCC has added the revised rule that a radiation power density within the RR prohibited band is −61.3 dBm/MHz, which is 20 dB less than the previous one, in the following Non-Pat. Document 2 in 2004, as a method for not depending on a side lobe of an antenna.
Non-Pat. Document 2: “Second Report and Order and Second Memorandum Opinion and Order” FCC 04-285, Dec. 16, 2004
In a conventional UWB radar, a system has been used in which a continuous wave (CW) from a continuous wave oscillator is turned on/off by a semiconductor switch.
In this system, a large residual carrier occurs due to the incompleteness in isolation of the switch. For this reason, as shown by the broken line in FIG. 21, the aforementioned residual carrier is evacuated into a short range device (SRD) band of 24.05 to 24.25 GHz which is allocated for a Doppler radar.
However, there is the serious problem that the SRD band is extremely close to the aforementioned RR prohibited band, which brings about unavoidable interference with the EESS or the like.
In order to solve the problem, there has been proposed a method in which a burst oscillator shown in the following Non-Pat. Document 3 is used for a UWB radar.
Non-Pat. Document 3: “Residual-carrier free burst oscillator for automotive UWB radar applications”, Electronics Letters, 28 Apr. 2005, Vol. 41, No. 9
The burst oscillator oscillates only when a pulse is in an on-state, and stops oscillation when a pulse is in an off-state. A residual carrier does not occur when such a burst oscillator is used for the UWB radar.
Accordingly, an arbitrary spectral array is possible, and a frequency band as shown by the solid line in FIG. 21 can be used for the UWB radar. As a result, it is possible to suppress a radiation power density within the RR prohibited band to be sufficiently low.
However, it is not easy to reduce by 20 dB or more in the above-described radiation power density from a spectral peak through sole use of the burst oscillator.
In this case, if the antenna has a characteristic with a sharp notch in gain within the above-described RR prohibited band, the UWB radar satisfying the new rule of the FCC can be realize by using this antenna in combination with the aforementioned burst oscillator.
The present invention is designed to provide such an antenna suitable for the UWB radar, the antenna having a notch in gain within the RR prohibited band.
As an antenna for satisfying these requests, first of all, it is necessary to realize a wideband thin, flat antenna.
As a thin, flat antenna, a so-called patch antenna configured such that rectangular or circular tabular antenna elements are formed in a pattern on a dielectric substrate has been known.
However, this patch antenna is generally a narrow band type, and in order for this to be a wideband type, it is necessary to use a substrate with a low dielectric constant, and to make a thickness thereof larger.
Further, a low-loss substrate is necessary for being used within a quasi-millimeter waveband, and Teflon (registered trademark) has been known as such a substrate.
However, because Teflon has a drawback in joining of metal films, it is difficult to manufacture an antenna, which brings about the problem of high cost.
In addition, as a wideband circularly polarized antenna, one in which spiral antenna elements are provided on a relatively thick dielectric substrate has been reported in the following Non-Pat. Document 4.
Non-Pat. Document 4: Nakano et al. “Tilted-and Axial-Beam Formation by a Single-Arm Rectangular Spiral Antenna With Compact Dielectric Substrate and Conducting Plane”, IEEE Trans. AP, vol. 50, No. 1, pp. 17-23 Jan. 2002
A spiral antenna is generally a balanced type antenna having a pair of spiral elements.
However, in the above-described Non-Pat. Document 4, the antenna is configured by one spiral element, which makes it possible to unbalanced feed that no use a balun.